This application claims the benefit of German patent application 101 12 423.6 filed Mar. 15, 2001, herein incorporated by reference.
The present invention relates to a cylinder for guiding at least one yarn and to a method for vibration damping a rotatably seated cylinder jacket of a cylinder. More particularly, the present invention relates to such a yarn-guiding cylinder basically comprising a hollow-cylindrical cylinder jacket, a support on which the cylinder jacket is rotatably seated, at least one magnetic bearing having a plurality of bearing pole windings distributed on the support circumferentially relative to the cylinder jacket, and a plurality of sensors for determining an actual position of the cylinder jacket, with the sensors and the bearing pole windings being coupled with each other by a control unit.
Cylinders of various types and shapes for guiding one or several yarns are employed in various textile applications, such as spinning installations or textile machines. For example, such cylinders are employed as godet rolls, which are used for conveying, drafting or heating of yarn. Cylinders are also known which serve as tension cylinders to conduct the yarn to a bobbin surface during winding of the yarn. It is common to all such cylinders that the cylinder jacket is rotatably seated for being operated at a circumferential speed which can essentially be equal to the yarn speed. The cylinder jacket may be seated on a support by means of bearings for this purpose. Cylinders in the form of godet wheels are known from European Patent Publication EP 0 770 719 B1 or German Patent Publication DE 197 33 239 A1, wherein the cylinder jacket is magnetically seated. At least one radially acting magnetic bearing is provided for this purpose, which bearing has a plurality of bearing pole windings distributed on the support. The bearing pole windings are evenly distributed over the circumference of the cylinder jacket. Sensors are associated with the bearing pole windings to monitor a bearing gap between the godet wheel jacket and the bearing pole winding. The sensors and the bearing pole windings are attached together on the support, so that the position of the cylinder jacket can be stabilized in relation to the support. However, with the known cylinders, the problem arises that externally produced resonances and machine vibrations cause the support to experience vibrations which have a negative effect on the control of the position of the godet wheel jacket. Under disfavorable conditions, there is even the possibility that the cylinder jacket is caused additional vibrational excitations, which lead to the instability of the bearing.
It is accordingly an object of the present invention to further develop a yarn guiding cylinder of the aforementioned type in such a way that the rotatable cylinder jacket is seated with as little vibration as possible.
It is a further goal of the present invention to prevent the danger of deformations of the cylinder as a result of sympathetic vibrations.
These objects are addressed by means of a yarn guiding cylinder of the basic type having a hollow-cylindrical cylinder jacket, a support on which the cylinder jacket is rotatably seated, at least one magnetic bearing having a plurality of bearing pole windings distributed on the support circumferentially relative to the cylinder jacket, and a plurality of sensors for determining an actual position of the cylinder jacket, with the sensors and the bearing pole windings being coupled with each other by a control unit. In accordance with the present invention, the sensors are fixed in place at a distance from the cylinder jacket and at a distance from the support for determining the actual position of the cylinder jacket in relation to a stationary reference position of the cylinder jacket.
The present invention further addresses these basic objects by providing a method for vibration damping of a cylinder jacket of a cylinder which is rotatably seated by at least one radially acting magnetic bearing, wherein the magnetic bearing is controlled as a function of an actual position of the cylinder jacket. Basically, the method of the present invention comprises the steps of:
a) determining the actual position of the cylinder jacket in relation to a stationary reference position of the cylinder jacket,
b) determining a chronological change of the actual position of the cylinder jacket as a function of a rotary frequency of the cylinder jacket,
c) determining a sequence of control signals for the magnetic bearing from the change of the actual position of the cylinder jacket at the rotary frequency and, via the control signals, causing bearing forces on the cylinder jacket counteracting the change in position of the cylinder jacket, and
d) controlling the magnetic bearing by the sequence of control signals at the rotary frequency.
The invention is distinguished in that the sensor arrangement and the actuating device are disengaged from each other. As a result, the vibrations in the support cannot have an effect on the position of the cylinder jacket. To this end, the sensors for determining an actual position of the cylinder jacket are fixed in place at a distance from the cylinder jacket and at a distance from the support in such a way that it is possible to determine the actual position of the cylinder jacket in relation to a stationary reference position of the cylinder jacket. In this case, the reference position of the cylinder jacket constitutes the position of the cylinder jacket which assures an optimal function of the cylinder during operation. Therefore, a position which is predetermined in a machine can be called the stationary reference position. In this case, there is also the possibility that the position of the cylinder defined in the machine changes in the course of operation, such as in connection with a tension cylinder, for example, which yields because of the increase in bobbin size. Here, each change of the position based on the function would constitute a stationary reference position. In this connection, it is important for the invention that the detection of the position of the cylinder jacket takes place independently of the reciprocal effect between the support and the cylinder jacket, and therefore independently of the bearing gap between the respective bearing pole windings and the cylinder jacket.
In order to allow a detection of the position of the cylinder jacket in connection with protrudingly seated cylinder jackets, it is suggested in accordance with an advantageous further development of the invention to fix the sensor in place by means of several protruding sensor supports. In this case, the end of each of the sensor supports is fixedly connected with the machine frame. A sensor, which has a defined position in relation to the stationary reference position of the cylinder jacket, is preferably fastened to the opposite protruding end.
A further embodiment of the invention is particularly advantageous in connection with godet wheels, wherein several yarns are guided on the circumference of the cylinder jacket. In this case, the sensor supports are arranged within an annular space between the cylinder jacket and the support, so that the exterior area of the cylinder remains completely free.
Here, the sensors and the bearing pole windings can be arranged together on a common bearing level, so that a very fast and precise positional regulation of the cylinder jacket by the magnetic bearing is possible.
However, it is also possible to arrange the sensors and/or the bearing pole windings on several adjacent levels. The arrangement of the bearing pole windings on several bearing levels has the advantage that a large bearing capacity and a particularly advantageous stiffness of the bearing is achieved. Such an arrangement is preferred in particular in connection with cantilevered cylinders. In this case, the bearing pole windings can be positioned on the support so they are angularly offset from bearing level to bearing level. Here, the sensors can be arranged on the respective bearing level or an adjacent level.
Basically, two different advantageous embodiments of the cylinder are possible for controlling the bearing pole windings. In a first variant embodiment, one of the sensors is assigned to each one of the bearing pole windings. In this case, a sensor signal from the sensor is converted by the control unit into a control signal for controlling the associated bearing pole winding. With this arrangement, the distance between the sensor and the cylinder jacket is directly measured and is fed to the control unit as the control signal. The difference between a set point distance representing the reference position of the cylinder jacket and an actual distance between the sensor and the cylinder jacket is used in this case as the quantity for forming the control signal.
However, there is also the possibility of detecting the actual position of the cylinder jacket by means of a group of sensors, which are advantageously arranged on one level. Then, the sensor signals are computed in the control unit to provide the actual position of the cylinder jacket, from which the control signals for triggering the respective bearing pole windings are formed. In the process, a conversion takes place in such a way that the bearing gaps existing between the bearing pole windings and the cylinder jacket receive a change required for achieving the end position.
A further alternative is constituted by a further development of the cylinder wherein the bearing pole windings are arranged in pairs on a bearing level, offset by essentially 180xc2x0. A sensor is assigned in this case to each pair of bearing poles, and the control unit converts the sensor signal from the sensor into a control signal for controlling the assigned pair of bearing poles.
In order to provide a seating of the cylinder jacket which is as vibration-free and stable as possible, it is proposed in accordance with an advantageous further development of the invention to seat the cylinder jacket by means of two radially acting magnetic bearings. Here, a group of sensors, each arranged on an adjacent level between the bearing levels, is assigned to each magnetic bearing. Thus, there is the possibility of measuring a chronological change in the position of the cylinder jacket, besides the position detection, which have an effect on the cylinder jacket because of vibrations.
By means of the magnetic bearings, it is possible in this manner to perform a vibration damping superimposed on the seating of the bearing jacket. To this end, each group of sensors is connected with a control unit, which performs an evaluation of the sensor signals in view of a position and vibration determination of the cylinder jacket. The control signals generated by the control unit are then fed to the respective bearing pole windings for seating correction and vibration damping.
The method in accordance with the invention for vibration damping of a rotatably seated cylinder jacket can be performed by means of a particularly preferred embodiment which has the advantage that bearing forces are generated by means of the magnetic bearings, to counteract an occurring vibration in amplitude and frequency. Thus, the damping effect becomes effective only in case of an occurring vibration. No damping is performed in operating states in which no relevant vibrations occur. In accordance with the method of the invention, the actual position of the cylinder jacket in relation to a stationary reference position of the cylinder jacket is detected. Simultaneously a chronological change of the actual position of the cylinder jacket is determined as a function of a rotary frequency of the cylinder jacket, which is converted into a sequence of control signals for the magnetic bearing. Here, the control signals cause the generation of bearing forces in the magnetic bearing, which counteract the vibration-caused position change of the cylinder jacket. The rotary frequency of the magnetic bearing is controlled by the sequence of control signals, so that the vibrations are suppressed immediately after they occur.
In an advantageous variant of the present method, the actual position of the cylinder jacket is detected at two measuring spots. There is then the option of detecting first and second order sympathetic vibrations. With the first resonance, the phase position of the chronological changes of the actual position would be the same at both measuring spots. But with a second order resonance, the phase position of the chronological change of the actual position of the cylinder jacket at the two measuring spots would be offset by 180xc2x0. One bearing point with a magnetic bearing is assigned respectively to each of the measuring spots, so that the magnetic bearings are respectively controlled by a sequence of control signals for damping the vibrations.
Four sensors, which are arranged on a level at 90xc2x0 offsets in relation to each other and distributed over the circumference of the cylinder jacket, are preferably used in one measuring spot for detecting the actual position, as well as the chronological change, of the actual position of the cylinder jacket. A high degree of accuracy of the positional determination and the vibration detection is achieved by means of this arrangement.
Several exemplary embodiments of the cylinder in accordance with the present invention will be described in greater detail the following disclosure, with reference to the attached drawings.